Background of LDH and the application for CO2 capture

https://doi.org/10.1039/C9DT03913A

“Although solid sorbents have gained a lot of attention and there is steady progress with the promise to replace liquid amines on the industrial scale,¹⁰ most of these solid sorbents have limited operating temperatures for capture (generally <100 °C).¹¹ Moreover, there is a need to focus on solid sorbents that operate at industrial flue gas emission temperatures (200–800 °C) such as those used in the cement, oil refinery, iron and steel industries.¹² Metal oxides, mixed metal oxides and alkali metal ceramics show promising CO₂ capture behaviour (0.5 to 16 mmol g⁻¹) under flue gas conditions.¹³ However, the progress on this kind of medium to high temperature solid sorbents is rather slow or limited compared to that of other types of solid sorbents. Physisorbents such as MOFs, COFs, zeolites and porous carbon-based materials show good CO₂ capture and cycling stability at low temperatures (<100 °C).^9d Other solid sorbents that work at medium to high temperatures (200–650 °C), such as CaO and alkali metal-based ceramics show good CO₂ capture capacities but lack long term cycling stability.^9b On the other hand, LDH-based MMOs show poor capture and cycling stability. Therefore, this work focuses on the development of improved novel LDH-based solid sorbents that can successfully operate at high temperatures under industrial flue gas CO₂ concentrations.

LDH-based MMOs are ideal candidate materials for capturing CO₂ under industrial flue gas conditions due to their (a) fast adsorption/desorption kinetics, (b) ability to operate from medium to high temperatures (200–800 °C), (c) wide compositional variability that can be tuned to obtain a high CO₂ capture capacity, (d) tolerance to moisture, and (e) being environmentally benign and economical.¹⁴ LDHs derive their structure from mineral brucite Mg(OH)₂ and are represented by the general formula [M²⁺_1−xM³⁺_x(OH)₂]^x+(A^−x/n)·yH₂O, where M²⁺ = Mg, Co, Ni, Ca or Zn, M³⁺ = Al, Fe or Ga, A = anion (organic or inorganic ions), 0.15 ≤ x ≤ 0.33 and 0.5 ≤ y ≤ 1.0.¹⁵ LDHs show various physicochemical properties and have been used in a wide range of applications.¹⁶ Thermal decomposition of LDHs generates MMOs that have basic characteristics.¹⁷ These MMOs have gained a lot of attention as pre/post combustion CO₂ capture sorbents.¹⁸ The basicity and porosity of these oxides can be tuned by varying the layered metal cations and interlayer anions.¹⁹ These MMOs present a theoretically high CO₂ capture capacity (17 to 34 mmol g⁻¹, depending on the composition), which can be exploited at medium to high temperatures. Unfortunately, they still have several limitations, including a low measured capture capacity (around 1 mmol g⁻¹), poor thermal and mechanical stability and particle aggregation during cycling.²⁰ Although there have been some efforts towards addressing these issues using various approaches,²¹ there has been no sufficient progress made to utilize these MMOs for large scale CO₂ capture.

It is very important to understand the crystal chemistry and physicochemical properties of LDHs to develop novel MMOs that show higher capture and a better cycling stability than those previously reported.²⁰ Thermal decomposition of LDHs leading to the formation of MMOs involves three different steps.¹⁷ In the first step, the adsorbed water is eliminated followed by a second step where the removal of intercalated anions and crystalline water molecules occurs. This step removes the interlayer space between the metal hydroxide sheets. Finally, in the third step, the dehydroxylation of the layered hydroxides takes place, leading to the formation of the MMOs. However, during the thermal decomposition step, the LDHs lose their interlayer space and layered structure leading to the formation of agglomerated MMOs (Scheme 1). These two factors have contributed significantly to the poor CO₂ capture performance of LDH-based MMOs reported so far. Therefore, it is important to synthesize LDHs in such a way so that they retain/transform the interlayer anion as a stable support for the resultant MMOs during thermal decomposition.”